(1) Field of the Invention
This invention concerns powder coating compositions and particularly powder coating compositions that have improved fluidization properties as a result of including a small amount of a metal oxide fluidization enhancer in the composition. This invention is also a powder coating composition that includes a small amount of a novel metal oxide gloss reducing agent.
(2) Description of the Art
Polymer based powder coatings are solid particulate compositions which are generally applied to substrates to give the substrate a tough durable surface coating. Powder coatings are applied to substrates by electrostatic, corona, or tribocharge spray process in which the powder coating particles are electrostatically charged by the spray gun and the substrate is grounded or oppositely charged. The applied powder coating is then heated to melt and fuse the particles into a continuous coating and to cure the resulting coating. The powder coating particles which do not adhere to the substrate can be recovered for re-use. Powder coating compositions are generally free of added solvents and, in particular, do not use organic solvents and accordingly are nonpolluting.
Powder coating compositions generally comprise a solid film-forming resin, usually with one or more coloring agents such as pigments. They are usually thermosetting, incorporating, for example, a film forming polymer and a corresponding curing agent (which may itself be another film-forming polymer). Powder coating compositions are generally prepared by intimately mixing the ingredients, for example in an extruder, at a temperature above the softening point of the film-forming polymer(s) but below a temperature at which significant polymerization would occur. The extrudate is usually rolled into a flat sheet and reduced in size, for example, by grinding, to the desired particle size. The particle size distribution required for most commercial electrostatic spray apparatus is between 10 and 120 microns, with a mean particle size within the range of 15 to 75 microns, and preferably 25-50 microns. Examples of powder polymer coating compositions are disclosed in U.S. Pat. Nos. 5,461,089, 5,470,893, 5,614,323 and 5,229,460.
Powder polymers cure to a glossy finish. In some applications, however, gloss reduction to gloss is desired. Gloss reduction is typically achieved in powder paints by adding large inert particles known as "texturing agents" to the compositions. The texturing agents become encased in the cured polymer film and are large enough to give the final finish a rough appearance. Examples of large inert texturing agents include fibers, pigments, mica and abrasives, such as those disclosed in U.S. Pat. No. 5,470,893.
Powder polymer coating compositions are also difficult to fluidize and to keep free flowing during their delivery, application, and use because of the tendency of powder polymer coatings to cake, become compacted, and to agglomerate during storage, reclaim, and handling. The problem with fluidization is becoming more predominant as the particle size of the powder polymer compositions decrease.
Traditionally it has been typical to use precipitated or gelled silicas as free flow additives, particularly to larger particle size powder coatings. Precipitated silicas and gelled silicas are believed to enhance the free flow of powder polymer compositions due to their ability to absorb moisture in the form of water vapor or liquid which minimizes liquid bonding and liquid-liquid bridging between the powdered polymer particles. However, with smaller and smaller powdered polymer particles becoming the norm, improved additives that adsorb moisture and/or that enhance powder polymer performance are needed.
While existing powder coating compositions are satisfactory in many respects, there nevertheless exists a need for powdered polymer coating compositions that produce a low gloss finish. It is an objective of the present invention, therefore to provide a reduced gloss powdered coating composition. There is also a need for new fluidization enhancer compositions that can be combined with powdered polymer compositions, and especially with small particle size powdered polymer compositions in order to enhance powder polymer performance.